The invention relates to a method for determining a speed of a vehicle.
In vehicles, the speed is normally measured indirectly via the wheel speed. In order to record the wheel speed, inductive transmitters or Hall sensors are commonly used. Inductive transmitters or speed sensors using the induction principle are passive sensors which supply a voltage signal proportionate to the speed. The speed sensor scans an impulse disc. Since the movement of the impulse disc is a prerequisite for the generation of the voltage by the sensor, the passive speed sensor permits no zero speed measurement. Hall sensors are active speed sensors which are based on the Hall effect or which use the magneto-resistance principle. These sensors can already emit a zero speed signal.
For both sensors, in borderline situations such as accelerating, braking or skidding, the speed measured indirectly based on the wheel speed does not coincide with the real speed on the ground. The real speed vector can only be inadequately determined via the wheel speed. Even when driving very slowly, as in “stop and go” situations, the speed is too imprecisely measured via the wheel speed. For driver assistance systems such as a traffic jam assistant, the precise speed is an absolute requirement, however, in order to correctly trigger the actuators.
On an optical basis, a system exists which is produced by CORRSYS-DATRON which senses the sub-ground using crossed grids, and which is able to determine to a very high degree of precision the real speed vector (VDI reports 1731, 2003, o.25ff, “Oitteroptische Sensorik zur berührungslosen Lãngsund uergeschwindigkeitsmessung”, (Optical grid sensors for non-contact longitudinal and transverse speed measurement) J. Haus, R. Schafer)—This system has been established on the market for reference measurements, but due to its high cost it is not used for measuring speed during standard driving operations.
The object of the invention is therefore to provide an improved method and a device which enable a speed of a vehicle to be determined at low cost.